Delete unused imports in spring-test

spring-test/src/test/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilderTests.java

@@ -19,7 +19,6 @@ package org.springframework.test.web.servlet.request;
 import java.io.IOException;
 import java.net.URI;
 import java.net.URISyntaxException;
-import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.security.Principal;
 import java.util.Arrays;